US and British troops were locked in fierce gunfights with Republican Guard soldiers yesterday as they struggled to take control of Umm Qasr, a small strategically important port on the Kuwaiti border.

The port will be used to bring in food and logistics supplies once fighting is over.

Military officials said 120 elite Republican Guard soldiers had been sent to the Kuwaiti border to halt the coalition advance.

"They were inserted to put some backbone into the troops, which they haven't done," said Air Marshal Brian Burridge, the commander of UK forces in Iraq. Only the Republican Guards were fighting now, he said. "At the moment they are fighting fiercely."

British marine commandos were sent in to reinforce the US troops yesterday.

Although US generals insisted the war was going to plan and that troops were advancing faster than expected, there was not the mass surrender that military planners had hoped for.

In many cases coalition troops have met unexpectedly strong resistance. As well as the fight at Umm Qasr, US troops talked of facing resistance at Basra, further north at Nassiriya and at the Shia religious town of Najaf, 100 miles south of Baghdad.

At Umm Qasr, remarkable live television footage showed US marines using heavy machine guns and Abrams M1 tanks as they tried to kill remaining groups of Iraqi soldiers. British Harrier jets were called in for air strikes.

Marines said that as soon as US forces crossed over a sand berm and a trench on the Kuwaiti border they came un der heavy fire. "We called in artillery from British units behind us but the explosions were coming in so close we were forced to pull back," said Sergeant Chris Demuro, 31, one of the marines in Fox Company who pushed forward at the front of the assault.

"It was terrifying. There was noise and confusion and we could see the white muzzle flashes from the guns of the Iraqis in a group of buildings up ahead."

US marines returned fire and two Cobra helicopter gunships checked the area before they pushed through. A blue Toyota truck appeared and headed straight towards them with a car behind it.

"We tried to flag it down but it kept on going and when it passed an Abrams tank, weapons were seen inside the cab. We were under threat and had no option but to stop it and a Humvee jeep blasted it with a 50-calibre machine gun.

"The bullets lit up the truck and in an instant it was a rolling fireball: no one inside it would have stood a chance."

The car following them was carrying a family, some of whom were injured and were later treated by the marines.

Sgt Demuro said Iraqis in civilian clothes surrendered and the marines found their uniforms and weapons hidden nearby.

At the dockyard by the port the marines spotted a lone gunman manning a sentry post above the main gates. He was promptly killed by a tank shell fired from 40 metres.

The marines then ran through the port buildings, marking each cleared room with a red cross on the wall. An Iraqi colonel was captured in one of the offices and was being questioned.

Dozens of Iraqis are believed to have died during the battle and at least one marine was shot dead by a sniper during the fight on Friday. At least 450 Iraqis surrendered.

At least 40 Iraqi soldiers were still holding out yesterday in the old part of the city.